The following devices are well known as carburetors that supply fuel to two-cycle or four-cycle general purpose engines: fixed-venturi carburetors that comprise a butterfly-type throttle valve and two fuel systems composed of a main system and a speed-reducing system as shown in Japanese Patent Application Laid-open No. 55-69748, and other publications; variable-venturi carburetors that comprise a cylindrical sliding throttle valve and a single fuel system in which the fuel delivery quantity is made variable by way of a metering needle as shown in Japanese Utility Model Publication No. 49-17682, and other publications; and carburetors that comprise a cylindrical rotating throttle valve and a single fuel system in which the fuel delivery quantity is made variable by way of a metering needle as shown in Japanese Patent Application Laid-open No. 58-101253, and other publications.
A carburetor comprising a single fuel system has well-known advantages in that the fuel passage structure is simple in comparison with one comprising a main system and a speed-reducing system, and the fuel consumption from low speeds to high speeds is smooth.
Conversely, means developed to supply a rich mixture required during engine start-up, and during low-temperature start-up in particular, include those for manually operating a start-up pump and introducing start-up fuel into the fuel system as shown in Japanese Patent Application Laid-open No. 55-69748, and other publications; and those for manually operating a throttle valve and increasing the aperture surface area of the nozzle aperture for delivering fuel that is adjusted by a metering needle as shown in Japanese Utility Model Application Laid-open No. 6-83943, and other publications.
When an engine to which fuel is supplied by the carburetor described above is operated, those components in the fuel passage extending from the constant fuel chamber and through the air intake passage of the carburetor that easily move due to engine vibrations may create inconsistencies in the fuel flow quantity. The weight of fuel in the fuel passage and in the constant fuel chamber may change due to the orientation of equipment mounted in the engine, creating inconsistencies in the fuel flow quantity. Inconsistencies in the fuel flow greatly affect the engine operating characteristics at start-up, warm-up, and other areas of operation.
However, drawbacks remain in the above-described conventional start-up fuel supply means because the feed rate may at times be inconsistent and at other times uniform, it is impossible to ensure stable warm-up even if start-up can be performed.